Glioblastoma multiforme (GBM) is an aggressive CNS malignancy, which is rarely curable. Diffuse brain invasion is a hallmark of this cancer. At the molecular level, the EGF receptor (EGFR) plays a central role in determining the physiology of many GBMs. EGFR gene amplification is common in GBM and frequently accompanied by an in-frame deletion of exons 2-7, which yields a truncated and constitutively active form of the receptor (EGFRvIII). Because of the profound effects of the EGFR and EGFRvIII on GBM cell physiology, EGFR-selective tyrosine kinase inhibitors (TKIs) and other EGFR-targeting therapeutics have been used to treat these tumors. Efficacy has been demonstrated; however, GBMs typically escape from control. We have identified the urokinase-type plasminogen activator receptor (uPAR) as a cell-signaling receptor that may become activated to support GBM cell growth and survival when EGFR signaling is neutralized or when GBM cells are treated with EGFR-targeting therapeutics. Activation of the uPAR cell-signaling system may increase GBM cell migration and invasion. In GBM cells, in which EGFRvIII is expressed, crosstalk pathways involving EGFRvIII and uPAR may be essential to activate the mitogenic transcription factor, STAT5B, and actualize a highly aggressive phenotype. Similar crosstalk pathways also may occur when the EGFR is amplified in the absence of EGFRvIII. The goal of this research project is to characterize the role of uPAR as a receptor that synergizes with the EGFR in GBM cells and as a receptor that may allow GBMs to escape from control in patients that are treated with EGFR-targeting therapeutics. Understanding uPAR mechanisms in GBM cells will facilitate rational design of uPAR-targeting therapeutics that could be used independently or in combination with EGFR-targeting drugs in patients with GBM. In Aim 1, we will study multiple model systems, including human GBMs that are propagated as xenografts, to test the hypothesis that activation of uPAR signaling constitutes an important pathway by which GBM cells escape from control by EGFR-targeting therapeutics. In Aim 2, we will characterize uPAR-EGFR crosstalk, at the molecular level in GBM cells, and test candidate therapeutic approaches for neutralizing this crosstalk. In Aim 3, we test whether inadvertent activation of the uPA-uPAR system, in GBM cells treated with EGFR-targeting therapeutics, induces phenotypic changes favoring cell migration and invasion. In Aim 4, we explore the relationship between uPAR and the EGFR in surgical specimens of primary and recurrent human GBMs. We also will apply quantum dot immunofluorescence microscopy to probe for evidence of uPAR signaling in human GBM specimens, at the single-cell level. Although this project has a basic science foundation, our objectives are highly translational. Ultimately, our goal is to complete the pre-clinical studies necessary to justify targeting uPAR for therapeutics development in GBM.